Electronic Supplementary Material (ESI) for Green Chemistry. This journal is © The Royal Society of Chemistry 2016

Supporting Information

Highly Efficient Formic Acid-Mediated Oxidation of Renewable Furfural to Maleic Acid with H2O2

Xiukai Li, Ben Ho, Diane S. W. Lim and Yugen Zhang*

Table of Contents

General Information ...... 1 Experimental Procedure ...... 2 Extended Optimization Tables...... 2 Identification of Reaction Intermediates...... 4

Reaction Pathways for H2O2 Oxidation of Furfural to Maleic Acid...... 7

General Information

Materials: all starting materials are commercially available and were used as received, unless otherwise indicated. Furfural (> 98%), formic acid (> 98%), and acetic acid (glacial) were purchased from Merck. Amberlyst 15, hydrogen peroxide (31%), propionic acid, and butyric acid were purchased from Sigma-Aldrich. The other chemicals not stated here were either purchased from Sigma-Aldrich, Merck, or Fisher Scientific. Product analysis: analysis of the reactant and product in the oxidation of furfural to maleic acid was performed using HPLC (Agilent Technologies, 1200 series) with UV (254 nm and 280 nm) and refractive index (RI) detectors. The reactant and product were separated on an Agilent Hi-Plex H column (7.7 × 300 mm, 8 µm) that was operated at 25 °C. The eluent was -1 a solution of H2SO4 (0.0001 M), flow rate 0.7 ml min , 25 °C. The retention times were respectively 8.5 min and 58.0 min for maleic acid and furfural on the UV detectors. The wavelength of 280 nm was chosen for the UV detector because furfural is more responsive at 280 nm than at 254 nm. The retention time was 12.9 min for formic acid on the refractive index (RI) detector. 1H and 13C NMR spectra were obtained using a Bruker AV-400 (400 MHz) spectrometer. Chemical shifts () are reported in parts per million (ppm) with reference to tetramethylsilane with the solvent resonance as the internal standard.

S1 Experimental Procedure

Furfural oxidation to MA: Furfural (96 mg, 1 mmol), formic acid (4 ml), and 31% H2O2 (1 ml) were charged into an around 8 ml thick wall glass tube. The glass tube was sealed and heated at 100 °C for 40 min. The glass tube was cooled to room temperature and an aliquot of the reaction mixture was analysed by HPLC to determine the MA yield (93%). The solvent of the reaction mixture was removed under reduce pressure to give a white solid product of MA (105 mg, 90% yield).

Extended Optimization Tables

Table S1. The conversion of furfural to maleic acid in different solvents.

Solvent Conv. (%) Yield (%) Acetic Acid 100 65 Water 99 15 1,4-Dioxane 74 0 MIBK 100 0 Toluene 100 0 DMSO 17 0 DMF 87 0 Acetonitrile 40 0 Ethanol 37 0 Ethyl Acetate 21 0 Reaction conditions: furfural (1 mmol), solvent (4 ml), 31% H2O2 (1 ml), 100 °C, 4 h.

Table S2. The oxidation of furfural in formic acid (FA) with different water content.

Entry H2O: FA (v/v) Conv. [%] Yield [%] 1 1:1[a] 100 28 2 1:1[b] 100 27 3 3:1[a] 100 13 4 3:1[b] 100 13 [a] Reaction conditions: furfural (1 mmol), solvent (4 ml), 31% H2O2 (1 ml), 100 °C, 1 h.

[b] Reaction conditions: furfural (1 mmol), solvent (4 ml), 31% H2O2 (1 ml), 100 °C, 4 h.

S2 Table S3. Furfural oxidation to MA in sealed and open systems.

T / °C t / h Vial Conv. (%) Yield (%)

Sealed 100 91 100 1 Open 100 88 Sealed 100 95 60 4 Open 100 84

Reaction conditions: furfural (1 mmol), formic acid (4 ml), 31% H2O2 (1 ml). A condenser was used for the reactions operated in the open system.

S3 Identification of Reaction Intermediates

Formic acid Maleic acid Acetic acid

1 Figure S1. H NMR (400 MHz, DMSO-d6) spectrum of the reaction mixture from furfural oxidation in acetic acid media with H2O2. Reaction conditions: furfural (1 mmol), acetic acid (4 ml), 31% H2O2 (1 ml), 100 °C, 4 h. After reaction some of the reaction mixture (0.1 ml) was dissolved in DMSO-d6 (0.7 ml) for NMR analysis. Formic acid,  = 8.126 ppm; maleic acid,  = 6.276 ppm; acetic acid,  = 1.933 ppm.

Maleic acid

Furfural

Figure S2. The HPLC chromatogram of the reaction mixture from furfural oxidation in formic acid media with H2O2. Reaction conditions: furfural (1 mmol), formic acid (4 ml), 31% H2O2 (1 ml), 100 °C, 40 min. After reaction some of the reaction mixture (30 mg) was dissolved in H2O (1 ml) for HPLC analysis. Retention time: maleic acid, 8.5 min; furfural, 58.4 min. Acetic acid was not observed by the UV detector (λ = 280 nm).

S4 HO OH

O O

1 H NMR (400 MHz, DMSO-d6)  6.288 (s)

Maleic acid DMSO-d6

13 C NMR (400 MHz, DMSO-d6),  130.50, 167.31

1 13 Figure S3. H and C NMR (400 MHz, DMSO-d6) spectra of the crude product from furfural oxidation in formic acid media with H2O2. Reaction conditions: furfural (1 mmol), formic acid (4 ml), 31% H2O2 (1 ml), 100 °C, 40 min. The solvent was removed by evaporation under reduced pressure and the obtained white solid was dissolved in DMSO-d6 for NMR spectroscopic analysis.

S5 Formic acid DCM

1 Figure S4. H NMR (400 MHz, DMSO-d6) spectrum of the isolated furan-2(5H)-one.  (ppm) = 4.96 (dd, J = 2.2, 1.7 Hz, 2H), 6.24 (dt, J = 5.8, 2.2 Hz, 1H), 7.91 (dt, J = 5.8, 1.7 Hz, 1H). Experimental procedure: furfural (1 mmol), formic acid (4 ml) and 31% H2O2 (1 ml) was stirred at room temperature (25 °C) for 1 h. The reaction mixture was diluted with 5 ml of water, and then extracted with 10 ml of dichloromethane (DCM). The DCM phase was separated and the solvent was removed under reduced pressure. The liquid product as obtained was subjected to 1H NMR spectroscopic analysis and furan-2(5H)-one was detected as the major component.

S6 Reaction Pathways for H2O2 Oxidation of Furfural to Maleic Acid

Scheme S1. The oxidation of furfural to maleic acid via the 2-furoic acid route (N. Alonso- Fagundez, I. Agirrezabal-Telleria, P. L. Arias, J. L. G. Fierro, R. Mariscal and M. Lopez Granados, RSC Adv. 2014, 4, 54960-54972).

Scheme S2. The oxidation of furfural to maleic acid via the Baeyer-Villiger oxidation of furfural to furanol formate ester (N. Alonso-Fagundez, I. Agirrezabal-Telleria, P. L. Arias, J. L. G. Fierro, R. Mariscal and M. Lopez Granados, RSC Adv. 2014, 4, 54960-54972).

S7